1. Field of Invention
This invention relates to mobile communications and more particularly to a method and system for selecting vocoder rates and transmit powers for communications over an air interface of a wireless network.
2. Description of Related Art
Wireless networks are an increasingly popular means of communications. People are using mobile stations such as cellular telephones so that they can readily stay in touch with others.
The wireless network typically includes a base station. The base station produces a radiation pattern defining a cell and/or sector in which the mobile stations operate. The radiation pattern allows for the base station to transmit and receive signals over an air interface of the wireless network. The base station may exchange the signals with a mobile switching center within the wireless network. The mobile switching center sets up and connects calls so that the mobile stations can communicate with a remote device.
A vocoder (voice coder) on the base station and on each of the mobile stations may generate the signals that are exchanged over the air interface. The vocoder encodes speech at a data rate into the signals that are transmitted and received over the air interface. Typically, if the vocoder encodes the speech at a higher data rate, the signals better represent the speech. Thus, the vocoder may select a data rate of 9600 bits per second (bps), so as to produce signals that accurately represent the speech. Conversely, the vocoder may select a data rate of 2400 bps so as to produce signals that marginally represent the speech.
The number of calls that the air interface can handle (i.e., call capacity) is determined by the data rate at which the vocoders encode the speech. For example, a wireless network having vocoders encoding at a high data rate, e.g., 9600 bps, would have a smaller call capacity than a wireless network having vocoders encoding at a low data rate, e.g., 4800 bps.
Simultaneous mode vocoders (SMVs) have been developed to increase the call capacity of the wireless network. The SMVs account for a type of speech when encoding speech. For example, when conversation is occurring, the SMVs encode speech at a full data rate of 9600 bps. When there is silence, the SMVs transmit at ⅛th the full data rate (1200 bps). As normal voice conversations occur in bursts of time, the average data rate of the SMVs is lower than that of traditional vocoders. Thus, the SMVs permit a higher speech quality at a lower average data rate. In turn, the lower average data rate increases the call capacity of the wireless network.
Speech encoding is not the only factor that determines the call capacity of the wireless network. The transmit power of the signals transmitted over the air interface may also influence the call capacity. For example, the call capacity of the wireless network may decrease as the transmit power of the signals exchanged over the air interface increases. Conversely, the call capacity of the wireless network may increase as the transmit power of the signals decreases.
The transmit power over the air interface is typically set in accordance with a frame error rate (FER). A frame defines a fixed number of bits that are transmitted over the air interface. The FER is a ratio of (i) a number of frames that has been received over the air interface that have at least one bit in error to (ii) a total number of frames that has been received over the air interface. The transmit power can be adjusted to increase or decrease the FER over the air interface. The adjustment of the transmit power may also result in an increase or decrease in the call capacity of the wireless network.